A new class of cytoplasmic mutants (mit- mutants) of yeast will be studied biochemically and genetically. The mutants have specific lesions in enzymes of the mitochondrial inner membrane that contain subunit proteins translated on mitochondrial ribosomes. The objectives of the genetic studies will be to localize and map the mit- mutations on the mitochondrial circular DNA. This will be done by several methods, including crosses of the mutants to a large collection of rho- testers, by recombinational mapping and three point crosses, and by heteroduplex mapping using restriction fragments of mitochondrial DNA. The objectives of the biochemical studies will be to establish which of the mit- loci code for structural and/or regulatory genes. In addition the mutants will be used to probe the assembly of the ATPase and of the respiratory enzymes.